Local equilibrium and Lambda polarization in high energy heavy ion collisions

TL;DR

This paper investigates how local equilibrium effects, specifically the thermal shear tensor, influence Lambda hyperon polarization in high-energy heavy-ion collisions, improving agreement between hydrodynamic models and experimental data.

Contribution

It introduces a new local equilibrium term proportional to the thermal shear tensor that enhances hydrodynamic predictions of Lambda polarization at high energies.

Findings

The thermal shear tensor significantly affects polarization predictions.

Inclusion of the new term improves agreement with RHIC and LHC data.

Feed-down corrections also impact polarization measurements.

Abstract

The polarization of the $\Lambda$ hyperon has become an important probe of the Quark-Gluon Plasma produced in relativistic heavy-ion collisions. Recently, it has been found that polarization receives a substantial contribution from a local equilibrium term proportional to the symmetric derivative of the four-temperature vector, the thermal shear tensor. We show that, at very high energies, this term can restore the agreement between the experimental measurements and the predictions of the hydrodynamic model, provided that the hadronization hypersurface is isothermal. We review the theoretical derivation of this new term, discuss numerical computations at RHIC and LHC energies, and compare them with the experimental data. We also present the effect of feed-down corrections.